The present invention relates to an ultrasound imaging apparatus and method thereof, more particularly, to an ultrasound imaging method and apparatus capable of multiple transmit/receive-focusing using a plurality of orthogonal Golay codes.
The ultrasound imaging apparatus is widely used in the medical field for displaying an image of a target object, such as a human body. Ultrasound signals are transmitted to the target object, and are then reflected from the target object, thereby forming the ultrasound image.
In order to transmit the ultrasound signals, the ultrasound imaging apparatus generally includes a transducer array which includes a plurality of transducers and a pulser for driving each transducer. Each transducer generates ultrasound signals in response to the pulse applied from the pulser. During transmission of the ultrasound signal, a timing point of the ultrasound generation at each transducer is controlled, thereby transmit-focusing the ultrasound signals at a predetermined position within the target object. In other words, the pulser pulses the respective transducers with different time delays so that the ultrasound signals reach a desired position within the target object at the same time.
The ultrasound signals reflected from the target object are received by the transducer array. The time for the reflected signals to reach the respective transducers is different depending on the location of the transducers. Therefore, in order to compensate for the time difference among the transducers, a beamformer applies and adds the delayed time, with respect to the reflected signals, which are received by the respective transducers, and generates receive-focused signals.
The power of the received signals is remarkably lowered when the ultrasound signal is passing through a highly dense medium, such as the human body. As a result, when the target object is located deep in the body, the desired information is difficult to obtain in the above-mentioned ultrasound imaging apparatus. Most of the ultrasound imaging apparatuses currently being used generate ultrasound signals by applying a pulse of short duration to the transducers. Increasing the peak voltage of the pulse may solve the problems due to the attenuation of the ultrasound signals. However, there is a certain limitation to increasing the peak voltage of the pulse since this may affect the internal organs of the human body.
Instead of increasing the peak voltage of the pulse, the average power can be raised. As a result, the Signal to Noise Ratio (SNR) is remarkably improved. This method is called xe2x80x9cpulse compressionxe2x80x9d, and is used, for example, in radar equipment. An ultrasound imaging apparatus using pulse compression employs a coded long pulse instead of the conventional short pulse.
In conventional ultrasound diagnostic systems employing short pulses of high voltage, the resolution of an ultrasound image is determined by the impulse response characteristic of the transducers used in the ultrasound imaging apparatus. However, in an ultrasound imaging apparatus using the pulse compression, the resolution is determined by the convolution between the transducers and the coded long pulse. Such ultrasound imaging apparatuses include a pulse compressor based on a correlator at an ultrasound receiving part so that the same results may be obtained as if the short pulse were transmitted. Accordingly, the SNR can be raised effectively by using a predetermined voltage that is relatively lower than the peak voltage of the short pulse used in conventional ultrasound diagnostic systems.
The performance of the ultrasound imaging apparatus using the coded long pulse is highly influenced by the code characteristic. The image quality is determined by the relation between the frequency characteristic of the code used and the frequency characteristic of the ultrasound transducers. Furthermore, the system performance greatly depends upon how the correlator-based pulse compressor (provided at the receiving part) is implemented to obtain, with a coded long pulse transmission, the same result as if a short pulse was transmitted.
Some codes are biphase codes, consisting of 1 and xe2x88x921 values only; some are arbitrary sequence codes, consisting of arbitrary values. One can easily construct the hardware for an ultrasound transmitter when using a biphase sequence code. Additionally, among the biphase codes, the Golay code has the characteristic that side-lobes in pulse-compressed output (as described above) are completely removed. There have been great endeavors to take advantage of this characteristic in ultrasound imaging apparatuses by using the coded long pulse.
FIG. 1 illustrates an ultrasound pulse transmission process in a conventional ultrasound imaging apparatus using Golay codes. For convenience of explanation, the drawing only exemplifies a Golay code including code sequence set (A1, A2) having length of L and M=2 and transmission to one focal point P.
In a first ultrasound transmission at one pulse repetition interval (PRI), all array elements 1axcx9c1h within a predetermined aperture of transducer array 11 transmit ultrasound signals with increased amounts of delay to an object so that first code sequence A1 has focal point P. Then, all array elements 1axcx9c1h receive signals reflected from the object.
In a second ultrasound transmission at a next PRI, all array elements 1axcx9c1h within the predetermined aperture of transducer array 11 transmit ultrasound signals with increased amounts of delay to an object so that second code sequence A2 has focal point P. Then, all array elements 1axcx9c1h receive signals reflected from the object.
An image of the scan line can be displayed by using the signals received from those two transmissions. The signals received from respective array elements 1axcx9c1h are first pulse-compressed, and then the selected amount of delay loaded thereto. Alternatively, the pulse-compression of the signals can be performed after obtaining the results of loading the selected amount of delay.
When the ultrasound is transmit-focused to a focal point with the use of a conventional biphase Golay code as described so far, the transmission must be performed as many times as the number of sequences included in one Golay code, i.e. M number of transmissions. Consequently, the frame rate is reduced by 1/M compared to a general pulsing technique using the short pulse. In other words, since the conventional ultrasound imaging apparatus using a Golay code performs the transmission as many times as the number of sequences included in one Golay code, the ultrasound image is formed by using the reflected signals after performing the transmission, and thus remarkably reducing the frame rate.
It is, therefore, an objective of the present invention to provide an ultrasound imaging method and apparatus which is capable of multiple transmit/receive-focusing by using a plurality of mutually orthogonal Golay codes, resulting in a reduction in frame rate and improvement of the performance of the ultrasound imaging apparatus.
According to an aspect of the present invention, an ultrasound imaging method includes the steps of: (a) storing M number of Golay codes, wherein M is a positive number greater than two, the M number of Golay codes includes M number of code sequences, and the code sequences of the Golay codes are orthogonal to each other; (b) transmit-focusing ultrasound pulse signals to a transmitting focal point, wherein the transmit-focusing is sequentially performed by the M number of groups of the Golay code sequences converted into the ultrasound pulse signals; (c) receiving signals reflected from the transmitting focal point in response to the transmit-focusing with respect to each of M number of groups; (d) receive-focusing the received reflected signals; (e) performing pulse compression with respect to the receive-focused signals; and (f) forming a B-mode image by processing the pulse-compressed signals.
According to another aspect of the present invention, an ultrasound imaging method includes the steps of: (a) storing an M number of Golay codes, wherein M is a positive number greater than two, the M number of Golay codes includes M number of code sequences, and the code sequences of the Golay codes are orthogonal to each other; (b) transmit-focusing ultrasound pulse signals to a transmitting focal point, wherein the transmit-focusing is sequentially performed by the M number of groups of the Golay code sequences converted into the ultrasound pulse signals; (c) receiving signals reflected from the transmitting focal point in response to the transmit-focusing with respect to each of the M number of groups; (d) performing pulse compression with respect to the received signals, wherein the pulse compression is performed once with respect to each stored Golay code; (e) receive-focusing the pulse-compressed signals; and (f) forming a B-mode image by processing the receive-focused signals.
According to still further aspect of the present invention, an ultrasound imaging apparatus includes: (a) means for storing M number of Golay codes, wherein M is a positive number greater than two, the M number of Golay codes includes M number of code sequences, and the code sequences of the Golay codes are orthogonal to each other; (b) a plurality of transducers for transmit-focusing ultrasound pulse signals to a selected transmitting focal point, wherein the transmit-focusing is sequentially performed by the M number of groups of the Golay code sequences converted into the ultrasound pulse signals, and the transducers receive reflected signals from the transmitting focal points in response to the transmit-focusing with respect to each group; (c) means for receive-focusing the received reflected signals; (d) means for performing pulse compression with respect to the receive-focused signals, wherein the pulse compression is performed once with respect to each stored Golay code; and (e) means for forming a B-mode image by processing the pulse-compressed signals.
According to yet another aspect of the present invention, an ultrasound imaging apparatus includes: (a) means for storing M number of Golay codes, wherein M is a positive number greater than two, the M number of Golay codes includes M number of code sequences, and the code sequences of the Golay codes are orthogonal to each other; (b) a plurality of transducers for transmit-focusing ultrasound pulse signals to a transmitting focal point, wherein the transmit-focusing is sequentially performed by the M number of groups of the Golay code sequences converted into the ultrasound pulse signals, and the transducers receive reflected signals from the transmitting focal points in response to the transmit-focusing with respect to each group; (c) means for performing pulse compression with respect to the received signals, wherein the pulse compression is performed once with respect to each stored Golay code; (d) means for receive-focusing the pulse-compressed signals; and (e) means for forming a B-mode image by processing the receive-focused signals.